The electrodeposition of various polyamic acids onto a metal substrate, followed by a heat cure, has resulted in successful polyimide film coatings on many types of electrical conductors, as taught by Martello et al, in U.S. Pat. No. 3,846,269.
It has been found, however, that the polyamic acid polymer does not electrodeposit as uniformly as might be expected on a non-polished, bare copper substrate. Also, the technique for electrodepositing polyamic acid polymer films is anodic, and the film coatings appear to accumulate soluble copper, possibly as the result of diffusion of copper ions into the resin layer during the electrodeposition process. Partly cured resin films show some green color due to such copper inclusions.
Full cure to the polyimide requires temperatures of about 275.degree. C or higher. When the resin is cured to this temperature, such copper containing polyimide insulation tends to lose some extensibility and adhesion and the insulation may crack when wound around relatively small bends. It is thought that the copper inclusions are directly responsible for these results.
Tanaka, in U.S. Pat. No. 3,547,788, recognized this problem of copper diffusion when electrocoating an aqueous solution of water soluble resins in ammonium salt form. He first washed the bare copper conductor, and then electrocoated with nickel in an aqueous nickel sulfate bath to a thickness of between 0.5 to 1 micron. After this, he electrophoretically coated the substrate with an ammonia or an amine salt of an electrolytic resin. The resulting coating was not a film but a porous precipitation layer of accumulated resin particles. This coating was not pin hole free and had to be coated a second time with an insulating paint composition.
There is a need then for an improved method of electrocoating polyamic acid polymers onto copper substrates which eliminates copper diffusion into the polymer acid polymer film layer, and also provides a pin hole free, uniform film in a one step coating operation.